The present invention relates to casting methods and more particularly to casting methods and moulds utilised with respect to forming single crystal turbine blades for turbine engines.
Operation of turbine engines is highly dependent upon operating temperature. Thus, it is desirable to provide high temperature creep resistant turbine blades in order to achieve such high operational temperatures. In such circumstances it is desirable for these turbine blades to be formed as single crystals which have higher creep resistance at higher temperatures than polycrystal equiaxed grain structured or columnar grain structured components.
In order to grow single crystals, solidification in the casting process should take place such that no other crystals are nucleated or grown. One particular way of accomplishing this single crystal solidification is by arranging for the interface temperature between the solid and liquid faces to be slightly lower than the melting point of the solid with the liquid temperature slightly increased beyond the interface. It will be understood that in such circumstances a temperature gradient is created whereby the latent heat of solidification is conducted through the solidifying solid crystal which in turn facilitates creation of the single crystal as desired. Generally the growth rate of the crystal is low so that the temperature of the liquid/solid interface can be retained below the melting point of the solidifying solid. Previously a so called helix or pig-tail single crystal selector has been used in order that competitive grain growth is reduced to a single grain at the pig-tail helix due to the turnabout nature inhibiting all but survival of one grain path during solidification. In short, the pig-tail or spiral grain selector suppresses new creation of stray equiaxed grains at and above the melt back position or solidification interface. The spiral grain selector essentially joins the seed surface to the component to effectively filter spurious grains from growing into the component.
Despite the use of a pig-tail or spiral helix grain selector, it is still possible for there to be secondary orientation scattering during directional solidification from the seed crystal. Such scatter can be due to mis-alignment of the seed crystal with respect to the desired component during fabrication of the casting mould or alternatively the use of the spiral helix grain selector which in turn introduces a tortuous path for solidification which results in a doubling back of the solidification front which in turn leads to an accumulated mis-orientation or scatter. Reduction or elimination of scatter will improve achievement of a single crystal structure.